That is an assertion that is incumbent upon those who make it to prove. It sounds though that they are working from the presumption that any claim of demonic activity must be a manifestation of mental illness because they apparently believe that there is no such thing as demonic activity. As you can see, that is a circular argument. One cannot prove that demonic activity does not exist by dismissing all claims of demonic activity to be symptoms of mental illness.
This is not to say that there are not individuals who mistakenly believe that they are victims of demonic activity when in fact they are mentally ill. Indeed, before an exorcism can be done, a candidate must first be examined by mental-health professionals to rule out mental illness as the cause. It simply means that not all claims of demonic activity can be dismissed as symptomatic of mental illness.
